Along the progress of electronic components in recent years, many types of flat panel displays have been developed, which include PDPs, liquid crystal display panels, electroluminescence panels, fluorescence display panels, electrochromic display panels, light emitting diode display panels and gas discharge type display panels. Among them, PDPs are attentioned as thin type and large sized flat panel type color displays. In a PDP, there are many cells formed between a front substrate used as a display surface and a rear substrate, and an image is formed by conducting plasma discharge in the cells. These cells are formed by partitioning by partition walls, and in order to control display state of each pixel forming an image, an electrode is formed in each pixel.
A PDP is constituted by a front glass panel and a rear glass panel, and the front glass panel and the rear glass panel are sealed together by a sealing material. In the front portion of the panel, the front glass plate, there are transparent electrodes, bus electrodes, transparent dielectric layer and a protection film, and in the rear portion, there are the rear glass plate, address electrodes, white dielectric layer, fluorescent layer and partition walls. Ultraviolet rays is commonly changed to visible light by a function of such a fluorescent member.
On the front glass plate of PDP, electrodes for producing plasma discharge are formed, and for these electrodes, narrow linear silver electrodes are employed for many cases. Such employment of silver electrodes causes a problem of so-called yellowing that is a problem that the front glass plate is discolored to be yellow when the electrodes are formed by baking. Namely, the discoloration of front glass plate of PDP to be yellow causes a problem that the balance of three primary colors of light is impaired. Further, there also occurs a problem that its visible light transmittance decreases. This yellowing is considered to be caused by a colloid condensation reaction in which silver ions (Ag+) become Ag0 by reduction of e.g. Fe2+ or Sn2+ present in the glass.
As measures for solving the above yellowing of front glass plate, there are known, for example, a flat panel display substrate (refer to e.g. Patent Document 1) wherein a surface on which metal electrodes are to be formed is polished to remove a reducing heterogeneous layer formed on the surface, a plasma display device (refer to e.g. Patent Document 2) wherein the amount of Fe2O3 is less than 2,000 ppm (0.2%) and metal electrodes are made of silver, and a process for producing glass substrate for image display device (refer to e.g. Patent Document 3) wherein control is made to reduce the reduction power in a float bath when the amount of Sn2+ in a glass substrate exceeds a tolerance.
However, although the method described in Patent Document 1 is effective, it is necessary to polish produced glass plates and its cost for polishing is enormous. Further, the methods described in Patent Documents 2 and 3 are not always effective to solve the problem of yellowing.                Patent Document 1: JP-A-10-255669        Patent Document 2: JP-A-10-334813        Patent Document 3: JP-A-2004-189591        